Beta-glucans are material attracting attention for the utility because of their excellent bioregulatory functions that have recently been analyzed, such as lipid metabolism improving action, intestinal regulatory action, blood sugar controlling action, antitumor effect, and immune enhancing action. Application of such material to a broad range of processed foods will bring extreme benefits, not only contributing to enhancement of functionality of processed foods (addition of value) but matching the expectation of contribution to public health maintenance. Beta-glucans occur in a variety of organisms, including microorganisms, basidiomycetes, and plants, chiefly constituting the skeleton of the organisms. Beta-glucans, for the most part, serve to make up cell walls. Beta-glucans are composed mainly of glucose polymers having at least two kinds of β-1-2,1-3, 1-4, and 1-6-D-glucopyranose bonds.
JP-T-2001-501996 studies β-glucans derived from grains and gramineous plants. Some β-glucans from grains and gramineous plants contain polyphenols, which can cause a coloration problem. Moreover, the β-glucans from these origins are expensive due to low original β-glucan contents, which puts a limit on applicability to foods.
Among microorganisms and basidiomycetes there are strains which secrete the same β-glucan out of fungi as their cell wall component under some cultivation conditions. The cell wall of microorganisms and basidiomycetes contain a large quantity of β-glucan.
Beta-glucan of microorganism or basidiomycete origin, including β-glucan secreted out of fungi by microorganisms or basidiomycetes, β-glucan harvested from microorganisms or basidiomycetes by isolation, extraction, purification or like means, cell wall components of microorganisms or basidiomycetes, and fungi per se, can or could be added to processed foods, for example, as follows. (1) Culture fungi collected from the culture of microorganisms or basidiomycetes are added directly to raw materials of processed foods. (2) Cell wall components of microorganisms or basidiomycetes separated from the culture and purified are added to raw materials of processed foods. (3) Beta-glucan extracted from fungi or separated cell wall components is added to raw materials of processed foods. (4) The supernatant liquid of a culture of microorganisms or basidiomycetes or β-glucan separated and purified from the supernatant liquid is added to raw materials of processed foods.
In view of the fact that many of β-glucans are polymers having a molecular weight of 10,000 or more, and some of them are sparingly soluble in water, it is very difficult to uniformly mix β-glucan with materials of processed foods by the method (1) or (2). Processed foods having β-glucan added thereto by the method (1) or (2) suffer from impairment of texture or reduction of commercial value, such as uneven baking.
The methods (3) and (4) are advantageous in that β-glucan can be incorporated into processed foods relatively uniformly and that the β-glucan content in processed foods can be adjusted freely. However, the extracted and purified β-glucan has problems arising from its high water absorptivity. If such β-glucan is added as such to, for example, a dough mix containing wheat flour as a main ingredient, and the mix is kneaded together with water, the β-glucan forms lumps to make non-uniform dough, which results in processed food products with reduced taste and texture and reduced quality. When β-glucan previously dissolved in water is added to a dough mix (mostly in powder form), the resulting β-glucan-containing foods can have β-glucan dispersed therein relatively uniformly. In this case, however, dissolving β-glucan in water needs much time, the aqueous solution takes on viscosity, and it is not easy to obtain a uniform aqueous solution. Accordingly, to dissolve in water is an impractical operation that impairs the workability on site.
It has therefore been awaited to establish a convenient process for producing processed foods in which β-glucan of microorganism or basidiomycete origin is uniformly dispersed and to develop such a β-glucan material.
Beta-glucans activating the immune system include plant cell wall components (see JP-B-62-6692 and JP-A-2001-323001), those present in the hymenia and the mycelia of basidiomycetes (mushrooms) (see K Sasaki et al., Carbohydrate Res., vol. 47, 99-104 (1976) and JP-A-5-345725), cell wall components of microbial fungi, and those secreted and produced out of fungi.
It is generally well known that the cell wall components of any microorganisms contains β-glucans and exhibit immune enhancement. Among them yeast fungi (see JP-A-54-138115 and JP-A-9-103266), lactic acid bacterial fungi (see JP-A-3-22970 and JP-A-10-167972), fungus of Aureobasidium (see JP-B-6-92441), etc. are known to be of high safety and high utility value as foods.
Microorganisms that are known to secrete and produce β-glucan out of fungi exhibiting immune system enhancement include the genus Macrophomopsis, the genus Alcaligenes producing curdlan (see Syokumotusen-i no kagaku, Asakura Shoten, 1977, 108), and Aureobasidium pullulans (see Agaric. Biol. Chem., 47(6), 1983, 1167-1172 and JP-A-6-340701).
The β-glucans present in the hymenia and the mycelia of basidiomycetes (mushrooms) have high immune enhancing activity, and some of them, exemplified by lentinan extracted from the hymenia of Lentinus edodes, have been made use of as medicines. In general, however, production of β-glucan by the hymenia and the mycelia of basidiomycetes (mushrooms) greatly varies depending on the growth or cultivation conditions, and it is necessary to separate the β-glucan by extraction. As a result, complicated and wide-ranging molecular species of β-glucan are found in the resulting extract, including both high-molecular-weight ones and low-molecular-weight ones, which means unstable quality.
Thus, stable production of β-glucan with constant high activity is the standing problem relating to basidiomycetes (mushrooms). On the other hand, the cell walls of microorganisms contain a large amount of β-glucan and are of interest as a source of β-glucan. Nevertheless, because the cell walls of microorganisms contain other components than β-glucan and are insoluble in water, they need an extraction operation similarly to basidiomycetes in order to recover highly effective, water-soluble β-glucan. It has therefore been an issue to establish a process for stably extracting β-glucan of constant quality.
In contrast, the fermentation method of producing β-glucan using microorganisms secreting and producing water-soluble β-glucan out of fungi with high immune enhancing activity is an extremely effective technology, making it feasible to obtain uniform, water-soluble, and high-activity β-glucan. Along this line, processes of producing β-glucan using a microorganism belonging to the genus Aureobasidium which is known to secrete and produce high-activity β-glucan out of fungi have been proposed.
Microorganisms belonging to the genus Aureobasidium are known, however, to secrete and produce pullulan, α-glucan, out of fungi when cultured using carbon sources commonly used in microbial cultivation, such as sucrose (see JP-B-51-36360 and JP-B-51-42199). This has made production of high purity β-glucan difficult (see JP-A-06-340701). Moreover, microorganisms of the genus Aureobasidium are also called “black yeast”. Black yeast produces melanin pigment, with which the fungi or the culture solution are pigmented in black, and so is the resulting β-glucan. Therefore, the produced β-glucan has seriously ruined product quality. To address this problem, it has been proposed to generate a mutant that does not involve pigmentation by mutagenic treatment and to produce polysaccharides including pullulan by using the mutant (JP-B-4-18835). However, no strain is known that is capable of secreting and producing β-glucan of high purity with good efficiency out of fungi (but incapable of secreting and producing pullulans out of fungi) completely without involving melanin production.
Culturing techniques for the production of β-glucan in which by-production of pullulan, regarded as impurity, during culturing is suppressed have also been studied. JP-A-6-340701 and JP-A-07-51080 propose a process in which pullulan production is controlled by pH adjustment or use of specific sugar as a carbon source thereby to provide β-glucan at high purity. The problem of this process for the production of β-glucan is that the operation is cumbersome because special culturing conditions should be set or that the medium is costly because a special carbon source should be used.
Accordingly, for the production of β-glucan by the use of microorganisms of the genus Aureobasidium, a strain is needed that produces no, or a limited amount of, impurity such as pullulan even when cultured using inexpensive saccharides commonly employed in microbial cultivation as a carbon source and that involves substantially no or limited production of pigment melanin during production of β-glucan, thereby secreting and producing high-activity and high-quality β-glucan with good efficiency out of fungi.